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Deceleration stumble

Heavyleft

Heavyleft

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Im at my ends with trying to diagnose this issue so Im going to throw it out there and see if anyone can give suggestions where to look or if anyone has experienced this.

Stats: 1987, full RV Morse TBI set up, 350, 700r4, 3-wire O2 sensor.

Problem: When decelerating, I get a stumble when it reaches 1K RPMs and when I get to a complete stop, it smooth's out at 600 RPM (normal). No issues accelerating or while driving, only when decelerating and under 1K RPM. It will drop to about 4-500 RPM then back up to normal when stopped. This happens when the engine is cold or hot.

I have replaced or checked EGR, EGR gasket, EGR solenoid, IACV, TPS, CTS, fuel pump, O2 sensor, 140A alternator, cap and rotor, ignition coil, plug wires. Fuel pressure is at 14 PSI.

There is no know vacuum leak that I can find.

Any help is appreciated. Thanks in advance.

-Mike
 
Does the VSS work properly? It somewhat sounds like what some setups do when VSS isn't working right. Does it continually vary idle speed, or ONLY immediately after deceleration and no other time?
 
dyeager535 said:
Does the VSS work properly? It somewhat sounds like what some setups do when VSS isn't working right. Does it continually vary idle speed, or ONLY immediately after deceleration and no other time?
Click to expand...

No, when its in Park or Im stopped at a light, it smooth's out as per normal operation, its only when decelerating and gets around 1K RPMs.
 
'87 the VSS is located on the back of the speedometer. If you have cruise, and it works, your VSS is working. May not be getting to the ECM, but it's working. :)

If you stab the throttle at idle and let RPM's drop on their own, it doesn't do it either?

What happens if you downshift to 2nd gear? I assume you are normally decelerating in 3rd or 4th.
 
dyeager535 said:
'87 the VSS is located on the back of the speedometer. If you have cruise, and it works, your VSS is working. May not be getting to the ECM, but it's working. :)

If you stab the throttle at idle and let RPM's drop on their own, it doesn't do it either?

What happens if you downshift to 2nd gear? I assume you are normally decelerating in 3rd or 4th.
Click to expand...

Stabbing the throttle at idle it drops RPMs as per normal operation, no stumble....that's one of the reasons its been hard to diagnose, Im unable to sit in my driveway and play around. it only happens while driving and in gear.

I haven't tried manually downshifting yet.
 
I'm thinking EGR weirdness or TCC sticking, although neither of which seems real likely based on what you've checked/replaced so far and how it acts. If the problem is 100% repeatable, disconnecting EGR solenoid and testing, then same with TCC, would eliminate either one as the cause very quickly.

I know the TBI setups don't act the same as TPI when VSS is not working, but that whole dip and recover RPM thing at idle does sound familiar.
 
dyeager535 said:
I'm thinking EGR weirdness or TCC sticking, although neither of which seems real likely based on what you've checked/replaced so far and how it acts. If the problem is 100% repeatable, disconnecting EGR solenoid and testing, then same with TCC, would eliminate either one as the cause very quickly.

I know the TBI setups don't act the same as TPI when VSS is not working, but that whole dip and recover RPM thing at idle RPM's does sound familiar.
Click to expand...


I appreciate the input. It gives me some more direction to look into.
 
Really barely educated guesses, but that is the good thing with repeatable problems, you know immediately if you got it or not, and unplugging stuff doesn't cost a dime, and is faster than actually testing the whole circuit, at least to narrow it down.
 
dyeager535 said:
'87 the VSS is located on the back of the speedometer. If you have cruise, and it works, your VSS is working. May not be getting to the ECM, but it's working. :)

If you stab the throttle at idle and let RPM's drop on their own, it doesn't do it either?

What happens if you downshift to 2nd gear? I assume you are normally decelerating in 3rd or 4th.
Click to expand...

Tried downshifting today. It stumbles less when shifting down to 2nd gear.
 
Yep, stumbling less means it's not a TCC issue. You can guess at EGR, outside of that I agree, time to data log.
 
heavyleft said:
Stats: 1987, full RV Morse TBI set up, 350, 700r4, 3-wire O2 sensor.

Problem: When decelerating, I get a stumble when it reaches 1K RPMs and when I get to a complete stop, it smooth's out at 600 RPM (normal). No issues accelerating or while driving, only when decelerating and under 1K RPM. It will drop to about 4-500 RPM then back up to normal when stopped. This happens when the engine is cold or hot.

-Mike
Click to expand...
Although Morse is great! It's a new TBI and has not had min air set correctly on your engine. Have also seen them where blades were not in sync = one closed more then the other.

eagle mark said:
Turn your throttle blade stop screw in 1/4 turn and see what happens?
Click to expand...
This will give an idea if this is the issue.

From there only data log will help. After a drive of data logging let the vehicle idle in gear for a minute then in park for a minute at end of log and I will be able to tell if it is a min air adjustment or if something else is wrong.
 
eagle mark said:
Although Morse is great! It's a new TBI and has not had min air set correctly on your engine. Have also seen them where blades were not in sync = one closed more then the other.

This will give an idea if this is the issue.

From there only data log will help. After a drive of data logging let the vehicle idle in gear for a minute then in park for a minute at end of log and I will be able to tell if it is a min air adjustment or if something else is wrong.
Click to expand...

Greatly appreciate the input. I will work on it this week getting all the necessary equipment. Ive been wanting to burn a chip anyways. Thanks again.
 
I apologize that I have not gotten a chance to update this thread yet but Ive fixed the problem.

I should have followed all the little signs and my gut from the start. It was the EGR valve.

I had taken it off, cleaned it and tested as per all internet instructions I could find and it looked to be operational but turned out it was not. It was probably getting stuck more than likely. Along with the fact that it seemed to be operational and that no matter which EGR valve you get from your local autoparts store, its always the wrong one and doesn't work...i.e....positive versus negative pressure EGR valves. Did some more digging and came up with a replacement and got an AC Delco one from Amazon (best price).
 
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